Medical Design Briefs - November 2022 - 34

Medical Lasers: When the Laser Application Requires
Utmost Safety
To ensure optimal results,
critical laser parameters
must be measured.
MKS - Ophir
North Logan, UT
The range of laser applications in the
field of medicine and aesthetics in medicine
is vast and varied. This diversity is
made possible by the numerous types of
lasers that have been developed or enhanced
over the past decades. Throughout
the process - from the designing of
new laser systems to their application on
humans - the parameters of the laser
beam must be tested repeatedly. The options
are copious, and developments in
this area are progressing rapidly.
n Measuring the Power or Energy
of the Laser Beam
In many applications, measuring the
laser's power (expressed in watts) or its
energy (in joules) is the first step to
checking whether the laser beam meets
the requirements. To measure the laser
power of continuous-wave lasers or the
laser energy of a single pulse in pulsed
lasers, one can use photodiode sensors
or thermal sensors. The thermal sensors
have a series of bimetallic junctions.
When the laser beam hits the sensor
surface, a temperature difference
arises between the two metals, which in
turn creates a voltage.
Since the connecting points are in
series, with the hot points located on
the inside and the cold ones on the outside,
a radial heat flow occurs across
the disc. The voltage generated is proportional
to the input power. Thermal
sensors can measure laser powers from
fractions of milliwatts to thousands of
watts. This method is frequently used in
quality testing, for example, to determine
the power of solid-state lasers
used for surgery, or for diode lasers
used in hair removal, as these are usually
operated at higher powers.
Photodiode sensors, on the other
hand, are suitable for lower powers ranging
from picowatts to just a few watts.
They are based on a semiconductor that
generates a current proportional to the
light intensity and that exhibits linearity
over a wide power range. High-quality
photodiodes often employ an additional
filter , which reduces the light intensity
and thus makes it possible to measure
higher powers. Photodiode sensors are
used in clinical settings by medical technicians,
for example, to check that a laser
is functioning correctly for ophthalmological
applications.
For measuring pulsed lasers, pyroelectric
sensors are usually best suited. They
utilize a pyroelectric crystal that generates
an electrical charge in proportion to
the heat absorbed. The two metallic surfaces
of the crystal accumulate the
charge and load a capacitor that is connected
in parallel with the crystal. The
pulse energy measured in this way is read
out, and then the next pulse is measured.
When equipped with special diffusers,
these sensors can also be used, for
example, to measure pulsed solid-state
lasers with higher power densities, such
as those used in dermatology.
Depending on the area of application,
multiple options are available for outputting
the measurement data from the
respective sensors. The results can be
sent directly to different handheld display
devices or exported to a connected
PC via a compact interface adapter.
Fig. 1 - With the SP932U, MKS Instruments introduces a new CMOS-based beam profiler. A proprietary algorithm
ensures accurate measurements over a wide wavelength range of 190-1100 nm.
34
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n Analyzing the Laser Beam Profile
Numerous medical applications require
other parameters to be known in
addition to the power or energy of a laser
beam. Especially for sensitive applications
in dermatology, ophthalmology or
surgery, it is essential that the beam adheres
to the specified profile. Camerabased
imaging technologies are usually
used here: CCD or CMOS cameras are
intelligently combined with optical components
and powerful software.
Today, high-quality, camera-based beam
profilers can even reliably measure tunable
lasers that are adjustable from UV to
IR. On the one hand, the camera resolution
should be taken into account: If, for
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Medical Design Briefs - November 2022

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